Many in the Republican Party are feeling depressed these days following a brutal week of media coverage, infighting and disappointing poll numbers for Donald Trump. What is even more infuriating is that Democratic rival Hillary Clinton is vulnerable and beatable, but if Mr. Trump continues on this path of self-inflicted wounds and fails to broaden his coalition, he will hand over the election to Mrs. Clinton in the fall.

Just this week, Mr. Trump picked a fight with fellow Republicans Speaker Paul D. Ryan and Sen. John McCain, holding back on his endorsement in their primary races. Mr. Trump expects loyalty and feels slighted by both politicians, who have been critical of him at times. Both Mr. Ryan and Mr. Trump need to stop acting as estranged relatives and realize that their goal is the same: defeating Mrs. Clinton. The two could become a powerful alliance — Mr. Ryan as the policy wonk and Mr. Trump as the master marketer of ideas to millions of new voters. While Mr. Trump’s most loyal supporters may love the in-house rumble, many traditional Republicans — the ones who will provide crucial grass-roots support in the states in the fall — fear that their nominee is firing at the wrong target.

So far, the polls show the Trump campaign has not been able to gain more momentum, creating more divisions within his own party rather than expanding his voter base. And his post-convention “bump” has turned into a dive: In the latest Fox News poll, just 78 percent of Republicans said they would vote for Mr. Trump, while 12 percent — almost one in eight — said they would vote for Mrs. Clinton. While Mr. Trump is performing well among independents, white men, veterans and the military, he needs stronger support from the GOP base, and from women and minorities in key swing states.

Seemingly every day we hear of another billionaire Republican or political operative who will either vote for Mrs. Clinton or leave the party because of Mr. Trump. The idea of supporting Mrs. Clinton is a travesty — she has openly embraced democratic socialism and a progressive agenda that is both anti-Republican and anti-growth. While Mr. Trump founders, Mrs. Clinton is relentlessly focused on rebuilding the Obama coalition of 2008 and 2012 while boosting her numbers among skeptical white men.

Mr. Trump loves citing polls at his rallies, so here’s hoping that the latest poll numbers serve as a serious wake-up call for the candidate and the campaign.

This campaign is becoming a contest of discipline, which so far Mrs. Clinton is dominating. Despite her constant and baffling lies about her private email server, the new revelations of the $400 million payment to Iran’s terror-sponsoring regime on the release of U.S. hostages earlier this year and her stale economic agenda, Mrs. Clinton has effectively steered the national conversation to Donald Trump’s temperament. She is the pathological liar, but still has gotten away with defining Mr. Trump as the “unfit” candidate.

Mr. Trump is most effective when he focuses on the dismal record of Mrs. Clinton and President Obama, while pushing his message of job creation, economic growth and national security. It’s a simple and logical formula, yet many Republicans, including myself, are baffled by his inability to stay on message whether he gives an interview or speaks at a rally.

Daily headlines contend that Mr. Trump’s campaign is “suicidal” and that an “intervention” is needed. The reality is that the Republican National Committee and the Trump campaign are already working seamlessly to strengthen the party’s ground game and raise money. Mr. Trump’s July fundraising numbers were impressive, with small donors making up over 60 percent of those contributing to his campaign. Thousands continue to flock to his rallies. The Democratic National Committee, by contrast, is in disarray following the hacking scandal that led to the exit of key high-level officials.

While the mainstream media, Mr. Obama and Mrs. Clinton are salivating at the prospect of destroying Donald Trump, hoping to dictate how the voters should think, Mr. Trump needs to seize this moment, steal a page from President Bill Clinton’s political playbook and re-energize his campaign as the country’s new comeback kid.

This election is far from over. Events can change the race. In the meantime, Mr. Trump needs to display some Politics 101-style message discipline, refocusing his energy and rhetoric on Mrs. Clinton’s shady record and Mr. Obama’s incompetence, while making the case that he is the right person for the job.

• Mercedes Schlapp is a Fox News contributor, co-founder of Cove Strategies and former White House director of specialty media under President George W. Bush.

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